Increasing evidence suggests that the renin-renin substrate reaction is regulated by factors other than the concentrations of enzyme and substrate. Partially purified human renin and renin substrate extracted from the plasma of each of eight human subjects were used to construct substrate-velocity curves comparing the rate of substrate cleavage in the whole serum of each individual with the rate in a corresponding system containing purified autologous substrate, or purified substrate plus a small amount of autologous serum. Linear regression analysis of the double reciprocal plots were used to compare the kinetic constants in paired experiments. Maximal reaction velocity (Vmax) was significantly lower (p greater than .05) when the reaction rate was measured in whole serum, in seven of the eight patients, while Km did not differ significantly, suggesting the presence of noncompetitive inhibition by human serum. The mean Michaelis constant in serum was 449 ng/ml while the average native substrate concentration of the five normal subjects was 629 ng/ml, an excess of less than twofold. The data suggest that plasma renin activity in man depends upon the concentrations of inhibitor and substrate, as well as upon the concentration of renin.